In traditional universal mobile telecommunications systems (UMTS) or similar radio air interfaces, the control signaling of a wireless transmit/receive unit (WTRU) that is not in a connected mode, (e.g., not using a cell-dedicated channel (DCH) in a UMTS wideband code division multiple access (WCDMA) system), is transmitted on a common downlink (DL) channel such as a forward access channel (FACH) carried on a shared physical resource such as a secondary common control physical channel (S-CCPCH). Acknowledgement signals are sent on a contentious common uplink (UL) channel, (e.g., a random access channel (RACH)).
Recent changes to standards enable transmission of the downlink FACH information over a high downlink shared channel (HS-DSCH), previously only used for high speed downlink packet access (HSDPA) transmissions in a Cell_DCH state. The same mechanism may in the future be applied to a higher layer or to user plane data.
These changes have left the RACH as the only available uplink channel to provide uplink feedback. The RACH is a limited capacity contentious channel with small payload that in addition requires a lengthy collision resolution process. The RACH terminates in a radio network controller (RNC), which requires Iub delays.
Another method that is allowed in recent changes (but will likely not be practiced) is transmitting feedback on a dedicated channel. However, any channel that requires a power reference using a power controlled dedicated channel will severely limit the number of users that can be signaled in this mode and is not desirable.
Because of the unfeasibility of RACH or dedicated channel usage, the recent changes assume no feedback at all except for possibly low rate, averaged signal strength measurements possibly sent prior to transmissions. In particular, there are no positive acknowledgement (ACK) or negative acknowledgement (NACK) or timely channel quality indicator (CQI) measurements.
Therefore, these changes rely on multiple transmissions to achieve the required signaling reliability. The number of transmission is configured to the desired reliability but correct reception can only be verified with a radio link control (RLC) or transmission control protocol (TCP) ACK (if configured). This results in higher latency and loss of efficiency.
Therefore, a non-contentious shared uplink channel that does not require a fixed power reference or accurate time reference would be desirable.